The present invention relates to an objective lens which is installed in an optical information recording/reproducing apparatus for recording information to and/or reproducing information from an optical disc complying with a predetermined standard. More specifically, the present invention relates to an objective lens made of resin (hereafter, frequently referred to as a resin lens) having a numerical aperture larger than or equal to 0.75 which is suitable for information recording/reproducing for a high-recording density optical disc, such as a BD (Blu-ray Disc).
There exist various standards of optical discs (CD, DVD, etc.) differing in recording density, protective layer thickness, etc. Meanwhile, high-recording density optical discs (e.g., BD), having still higher recording density than DVD, are being brought into practical use in recent years to realize still higher information storage capacity. Incidentally, in this specification, the “optical information recording/reproducing apparatuses” include apparatuses for both information reproducing and information recording, apparatuses exclusively for information reproducing, and apparatuses exclusively for information recording.
Since the high-recording density optical disc has the higher recording density than DVD and CD, a beam spot having a relatively small diameter is required for information recording and reproducing for the high-recording density optical disc. That is, a relatively large NA is required for information recording/reproducing (i.e., information recording and/or information reproducing) for the high-recording density optical disc. For this reason, in general a glass lens having a relatively high refractive index is used as an objective lens for the optical information recording/reproducing apparatus as described in Japanese Patent Provisional Publication No. 2005-156719A (hereafter, referred to as JP2005-156719A). However, a glass lens has a drawback that since a glass lens is heavy in weight, the glass lens places a burden on operation of an actuator for driving the lens when the actuator moves the lens in a focusing direction or in a tracking direction.
Therefore, a designer might consider forming a lens with resin in place of glass as material in order to reduce weight of the lens. By achieving the weight reduction through use of the resign as material of the lens, it is possible to reduce the burden to be placed on the actuator for driving the objective lens. Furthermore, forming the objective lens for the high-recording density optical disc with resin provides advantages in regard to easiness of production, a high degree of productivity and cost. An example of a resin objective lens is disclosed, for example, in Japanese Patent Provisional Publication No. 2007-179720A (hereafter, referred to as JP2007-179720A).